1. Field of the Invention
The present invention relates to a light-emitting element, and more particularly, to a high-efficiency light-emitting element.
2. Description of the Prior Art
The applications of light-emitting diodes are extensive and include optical display devices, traffic signals, data storing devices, communication devices, illumination devices, and medical apparatuses.
The conventional light-emitting diode includes a substrate, a first electrode semiconductor layer formed on the substrate, a light-emitting layer formed on the first electrode semiconductor layer, a second electrode semiconductor layer formed on the light-emitting layer, and a transparent conductive layer formed on the second electrode semiconductor layer. LED light travels in each direction instead of focusing on one place. However, the light generated from an LED is not easily emitted from the LED. According to Snell's law, only light emitted at an angle within the critical angle θc would be completely emitted out, and other light would be reflected and absorbed. In other words, the angle of LED light must be within a cone of 2θc for light to be completely emitted out. Light emitted at an angle larger than 2θc is reflected. The top surface of the conventional light-emitting diode is often a planar structure, so the critical angle is very small and most light would be reflected. The reflected light is absorbed by the semiconductor material so that the light-emitting efficiency is reduced. Therefore, it is important to prevent the light from reflecting completely so as to increase the light-emitting efficiency.
The conventional solution to solve the above-mentioned problem is to form a microlens on the top layer of the LED so that the critical angle can be increased and most light would be completely emitted so as to increase the light output of the LED. The other solution is to apply photonic crystal technology to overcome the total reflection effect so as to increase the light-emitting efficiency. However these prior arts techniques require complicated manufacturing processes, which increases both the difficulty of the LED process and the cost. Furthermore, in theese prior arts the semiconductor layer with a certain thickness on the light-emitting layer is required as a window layer or a contact layer, and the light emitted from the light-emitting layer to the semiconductor layer would be partially absorbed by the semiconductor layer. So these prior arts can solve the problem of the total reflection but they cannot overcome the light absorption of the semiconductor layer.